The chlorinated pyridine derivatives of the present invention are known compounds, having been previously prepared by a number of known processes, and are also employed as chemical intermediates in the preparation of other highly desired herbicide or pesticide products. Previous methods for preparing such compounds include those described in U.S. Pat. Nos. 3,538,100 and 3,186,994 and the prior art noted therein. Thus U.S. Pat. No. 3,538,100 describes the preparation of 2,3,5,6-tetrachloropyridine and pentachloropyridine by chlorination of liquid 2,6-dichloropyridine at a temperature of at least 180.degree. C. and in the presence of a metallic halide catalyst. However, reaction times of 45-90 hours are required. U.S. Pat. No. 3,186,994 describes the preparation 2,3,5,6-tetrachloropyridine and pentachloropyridine by chlorinating, in the absence of a catalyst, a polychloro-(trichloromethyl)-pyridine reactant in the liquid state. However, the preferred reactions require either irradiation with ultraviolet light or temperatures above 400.degree. C.
U.S. Pat. No. 3,420,833 describes a process for preparing polychlorinated pyridines by chlorinating pyridine in the gas phase in the absence of a catalyst. However, as shown in comparative examples 29-33 hereinafter, the process described therein results in the predominant formation of 2,3,4,5,6-pentachloropyridine or the formation of pentachloropyridine with significant amounts of undesirable 2,4,6-trichloropyridine and/or 2,3,4,6-tetrachloropyridine co-product.
U.S. Pat. No. 3,370,062 describes the preparation of pentachloropyridine by chlorinating in the gas phase a mixture of pyridine, methylpyridines, and chloropyridines in the presence of catalysts such as silica or alumina. However, this reaction also results in the predominant formation of 2,3,4,5,6 pentachloropyridine and affords a yield of less than fifty percent. In addition, this reaction was not adaptable to form 2,3,5,6-tetrachloropyridine.
U.S. Pat. No. 3,555,032 describes a continuous process for preparing 2,3,4,5-tetrachloropyridine by chlorinating 2-chlorpyridine hydrochloride in the liquid phase. However, a reaction time of at least an hour and preferably 60 to 40 hours is needed, and this process does not yield the highly desirable 2,3,5,6-tetrachloropyridine.
U.S. Pat. No. 4,256,894 describes a process for preparing polychloropyridines by chlorinating chloro-substituted (trichloromethyl) pyridine in the liquid phase in the presence of a Lewis acid type catalyst. However, this process requires high pressure and a reaction time of 18-24 hours while affording a mixture of products.
U.S. Pat. No. 4,281,135 describes a process for preparing 2,3,5,6-tetrachloropyridine and pentachloropyridine by chlorinating 2,6-dichloropyridine in the liquid phase in the presence of an oxide catalyst. However, this reaction must be run under pressure with a reaction time of 11-30 hours.
Recently DE No. 3,306,905 reported the continuous preparation of pentachloropyridine by the gas phase chlorination of pyridine in the presence of coke as a catalyst. However, this process requires a fluidized bed reactor at a temperature of 550.degree. C.